1. Field
The present invention relates to the field of determining the parameters of particles, droplets or the like, and more particularly, to the determination of particle size and velocity using laser light scattering.
2. Art Background
There has long been a need to measure particle and droplet size in sprays, planetary atmospheres, combusiton processes and the like. Such measurements are useful in aircraft icing studies, planetary studies, fuel analysis, and numerous combustion and nozzle applications. A number of techniques employing laser light scattering have been developed to determine the size and/or velocity of particles, droplets or the like.
In one system, commonly referred to as a particle sizing interferometer, a pair of laser beams of equal size and intensity are caused to cross and form a sample volume. An interference pattern between the beams is established by this crossing, and particles moving through this volume scatter light in proportion to the spacially varying light intensity within the interference pattern. From the scattered light, information concerning both the particle size and velocity can be computed. The particle's size is determined from a visibility factor (V) which is a function of the relative maximum and minimum intensity of the received scattered signal. A more detailed explanation of this interferometric technique is given in "On Line Particle Monitoring Instruments", by Bachalo, Geffken and Weth, 1978 Symposium on Instrumentation and Control for Fossil Demonstration Plants, June 19-21, 1978; and U.S. Pat. No. 4,329,054, which issued on May 11, 1982. However, the determination of particle size based on a visibility function may give rise to significant errors. In the case where D/.delta. (where D=the particle diameter and .delta.=the fringe spacing) is in the range of 0.08-0.24, a 10 percent error in determining the visibility factor potentially results in a 300 percent size error.
The correlation between scattered light intensity and particle size has long been used for sizing small particles carried in a fluid. Various techniques have been devised in an attempt to utilize the intensity of the scattered signal as a direct measurement of particle size. These techniques are generally intrusive, in that that samples are drawn into the sizing apparatus and cannot be used to measure external sprays or the like. However, there exist non-intrusive techniques which utilize probability inversion methods to convert the apparent size of particles based on their absolute intensity into actual particle sizes. In addition, the particle's velocity may be determined from the fringe spacing of the detected interference signal. See, Yule, Chigier, "Particle Size and Velocity Measurement by Laser Anemometry", 15th Aerospace Sciences Meeting, (AIAA, Jan. 24-26, 1977). However, systems of this type base the determination of particle size on a number of assumptions used in the development of the probability function, in order to convert the apparent particle size into an "actual" particle size.
Since most laser beams have Gaussian intensity profiles over their cross section, a particle traversing the middle of a beam will scatter more light than if the particle crosses through the outer portion ("tail") of the beam. Therefore, in order to achieve a true intensity versus size correlation it must be determined if a certain scattered signal level corresponds to a small particle crossing through the central portion of the beam, or large particle crossing through the tail. Inasmuch as systems such as that disclosed by Yule and Chigier cannot determine if a particle has passed through the central portion of the laser beam, their analysis can only provide a statistical distribution of the size and velocity of the many particles which pass through their device.
Moreover, the sensed information from the laser interference pattern may represent scattering associated with multiple droplets. In the present invention, this multiplicity is significantly less of a problem than in prior art methods (for example visibility based systems) which rely on the quality of the phase front of the laser beams. A number of systems have been developed to insure that only single particles, droplets or the like passing through the central portion of the laser beam interference pattern are analyzed (See U.S. Pat. No. 4,329,054). However, these systems do not preserve the information needed in order to determine a particular particle's size and velocity simultaneously.
As will be described, the present invention overcomes the disadvantages associated with prior art systems, and discloses a technique which insures that the parameters associated with a particle are determined only if the particle lies within the central portion of the laser beam for any given measurement. In addition, the present invention provides a means whereby both the size of the particle, droplet or the like as well as its velocity may be simultaneously determined.